In general, a cryogenic refrigerator includes: an expansion type refrigerator unit accommodating a thermal accumulation material and has an expansion chamber located within the refrigerator; and a compressor unit containing a compressor main body. The refrigerator unit is installed within an apparatus or a container which is to be cooled to an extremely low temperature. Then, a high pressure refrigerant gas obtained through the compressor unit is fed to the refrigerator unit where the high pressure refrigerant gas is cooled by the thermal accumulation material and then expanded, followed by carrying out a further cooling step. Subsequently, a low pressure refrigerant gas is returned to the compressor unit, thereby forming a refrigerating cycle and thus obtaining an extremely low temperature by repeating such refrigerating cycle.
Conventionally, when such a refrigerator is used to perform temperature adjustment, an electric heater is provided in the refrigerator unit so as to introduce a thermal load and thus perform temperature adjustment.
However, since the heater is used in an extremely low temperature environment, its reliability is low, resulting in a low insulation which causes an electric leak and hence some troubles such as an emergency shut down due to such an electric leak.
Further, as another method, as recited in Japanese Patent Laid-Open Publication No. 2000-121192, it is conceivable that an inverter controls the rotation speed of a compressor main body to adjust a gas amount so as to effect temperature adjustment. Although this method is effective when a single refrigerator unit is operated by a single compressor unit, when a plurality of refrigerator units are operated by one or more compressor units, there had been a problem that it was impossible to perform the temperature adjustment of the respective refrigerator units.
Moreover, in the case where a plurality of refrigerator units are operated by one or more compressor units, since the valve timing at the start of each refrigerator unit is not changed, there had been a problem that an irregularity occurred among the flow rates of gases flowing into the respective refrigerator units (when intake timings got overlapped, more gas would flow to refrigerator units whose intakes occurred earlier), causing an irregularity among the refrigerating abilities of the refrigerator units.